This invention relates to a device for measuring the volumetric flow rate of a fluid in a hydraulic circuit. Various types of devices for measuring the volumetric flow rate of a fluid are available commerically. Such devices, and in particular those used for measuring the volumetric flow rate of a lubricant fluid, have a number of drawbacks the most serious of which is that they do not allow the volumetric flow rate of the fluid through the circuit to be accurately measured. Futhermore, many of these devices are very sensitive to the viscosity of the fluids passing through them, to the extent that for a given flow rate the indicated measurement can vary widely with change of fluid or rather its viscosity. Finally, many of the known devices easily develop mechanical faults which often prevent their proper operation.